mtd: Fix dependencies for !HAS_IOMEM archs

Not every arch has io memory.
So, unbreak the build by fixing the dependencies.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>